LIME AND CEMENT

Lime is calcium oxide with traces of magnesium oxide and ii is obtained by burning of naturally occurring lime stone (CaCo3).

CaCo3 -à CaO + Co2

Classification

Lime is classified on the basis of its calcium oxide content and its sources.

1. Fat lime or high calcium lime

High calcium lime contains 95-98% of calcium oxide and the balance being impurities like silica, alumina, magnesia etc., they are produced by burning of lime stone. In the absence of water or moisture, it hydrates rapidly in to powdery form of calcium hydroxide with the evolution of large amount of heat. This property is called slaking and high calcium lime has high slaking powder. They are non-hydraulic (slaking and setting in absence of water). Since the volume increases on slaking, it loses strength. As they are highly porous in nature, they have maximum shrinkage. Fat limes are usually used for white washing, water softening and in glass and metallurgical industries etc.

2. Lean lime or Poor calcium

Lean lime is the impure form of lime and it contains about 70-80% calcium oxide, 5% magnesium oxide and balance being impurities like iron oxide, silica etc., they slake slowly compared to high calcium lime. They are less porous in nature, and hence have medium shrinkage. They are mainly used for making lime mortar for plastering purpose as they have more plasticity and workability.

3. Hydraulic lime

They are right quality lime among different classes of lime. They contain about 70-80% pure calcium oxide and 5-30% impurities like silica, alumina, magnesia, iron oxide etc., they are highly hydraulic in nature as they become hard and get set in contact with water. They are non-porous in nature and hence, have very little shrinkage. They slake with difficulty.

Hydraulic lime is again subdivided in to three types based on the percentage of clayey impurities.

(a)Feebly hydraulic (5-15%)

(b)Moderately hydraulic (15-220%)

(c) Eminently hydraulic (20-30%).

The hydraulic property of lime is due to the presence of these impurities.

4. Dolomite lime

Dolomite lime is obtained by large scale calcinations of dolomite. The dolomite is an equimolar mixture of CaCo3 and MgCo3. They contain about, 30-40%magnesium oxide and traces of other impurities. It slakes slowly due to the high contents of magnesia. They are mainly used as fluxes in metallurgical operations.

MANUFACTURE OF LIME

Lime is obtained by the large scale calcinations of natural lime stone at about 1000-1200 degree Celsius.

CaCo3 ->reversible reaction

The process is carried out in vertical kilns of height about 40 m and 1.5-2m diameter. Inside of the kiln is lined with refractory bricks. Lime stone is fed in to the kiln through the top hopper. Heating is carried out by burning coal or producer gas at the fire box provided at the bottom of the kiln. The flue gas which include carbon dioxide, escape through the outlet at the top. During the burning process decomposition of lime stone produces lime (CaO). The air supply provision at the bottom of the kiln helps both in the calcinations of the lime stone as well as cooling of the produced hot lime at the bottom. The lime is finally collected in trolleys run under the kiln.

PROPERTIES OF LIME

1. Slaking property

It is the property of lime in contact with water. It absorbs the water and converts into a powdery form of calcium hydroxide with evolution of large amount of heat with a hissing noise.

CaO + H2O à Ca (OH)2 + 15.5 Kcal/gm. mol

2. Hydraulic property

It is the setting and hardening of lime when it in contact with water or moisture due to some chemical reaction between the constituents. It involves the following reactions.

(a)Dehydration of slaked lime of evaporation

Ca (OH)2 -à Cao +H2O

(b) Carbonation: The calcium ion produced to some extent and the calcium by dioxide absorbing the carbon dioxide from atmosphere, from crystalline calcium carbonate.

CaO+Co2àCaCo3 (crystalline)

Ca (OH)2 + Co2 àCaCo3 + H2O

The colloidal gel of calcium hydroxide and crystalline calcium carbonate finally form a hard mass. The clayey impurities form complex silicates and also crystallize to form hardness.

3. Air slaking- When lime is exposed to dry air, it absorbs carbon dioxide from the atmosphere and gets converted into air slaked lime, calcium carbonate.

CaO + Co2 -àCaCO3

PLASTER OF PARIS

Plaster of paris is prepared by heating pure gypsum CaSO4.2H2O to a temperature range of 120-160 degree Celsius. If it is heated above 200 degree Celsius, anhydrous calcium sulphate is formed

CaSo4.2H2O àßreversible reaction CaSo4.1/2H2O--àCaSo4

When plaster of paris is mixed with water, it quickly hardens and attains crystalline structure of gypsum. The hardening of gypsum can be accelerated by potassium sulphate or alum. Because of its quick hardening and setting property, plaster of paris is used for making moulds, surgical bandages. Its main use is for the manufacture of plaster boards for decoration purposes.

CEMENT

Cement is finely divided powder material having hydraulic property (unsetting and hardening). When mixed with water it is capable of binding building material like stones, bricks, building blocks etc.,

Different types of cement

1. Natural cement: It is obtained by the calcinations of natural lime stone containing 20-40% clayey impurities and subsequent powdering of the calcined mass. The hydraulic property of natural cement is due to the chemical reactions involving the formation of crystalline silicates and aluminates(clayey impurities) of calcium.

2. Puzzolana cement: it is one of the oldest cement materials. It is obtained by mixing powder material of natural Puzzolana (which is natural material of volcanic lava and ash) with lime without heating. It shows the hydraulic property as volcanic ash and lava is the main source of silicates and aluminates and other minerals.

3. Slag cement: The slag obtained from the metallurgical process (blast furnace slag) contains aluminates and silicates of calcium which is subsequently dried and powdered and then mixed with slaked lime, to produce slag cement.

4. Portland cement: Portland cement is the ordinary commercial variety of cementing material used for construction purposes. This variety of cement was discovered in 1824 by Joseph Aspdin. This cement when mixed with water and on setting and hardening, becomes a hard mass resemble in color and hardness to Portland stone and the name.

MANUFACTURE OF PORTLAND CEMENT

Portland cement is manufactured by the calcinations of proportionate mixture of calcium containing raw material called calcareous raw material, at about 1500-1600 degree Celsius and the calcined mass is ground well and a proportional quantity of powdered gypsum is added.

1. Raw material

(a) Calcareous raw materials e.g., lime stone. Chalk etc.,

(b) Argillaceous raw material e.g. Clay, slate.,

(c) Gypsum (CaSO4.2H2O)

2. Approximate composition of different constituents.

Lime CaO 60%

Silica SiO2 22%

Alumina Al2O3 5%

Magnesia MgO 4%

Gypsum CaSO4.2H2O 4%

Iron oxide Fe2O3 3%

Sulphur trioxide So3 1%

Alkali oxides Na2O/K2O 1%

3. Processes

Manufacturing of Portland cement generally involves the following process

Dry process: The calcareous and argillaceous raw materials are ground well into fine powder in a tube mill separately and mixed with equivalent proportions to get a raw mix and stored in storage tanks called silos. This raw mix is directed to a hopper through flow channel, for burning processes. This process is of less significance.

Wet process: The calcareous raw materials are ground and powdered well in a tube mill and stored in storage tank called silos. The argillaceous raw materials are washed in separate wash mills to aggregates organic matters .Mixing of raw materials by Dry process and Wet process matters and other unwanted impurities and it is stored in big basins. The calcareous raw materials from silos and washed argillaceous raw materials from basins are taken discharge to flow channels, in proper proportions, which lead again to grinding mills. There it is thoroughly mixed with water to form slurry which is then sent to furnace process.

(b)Furnace operations

1. Rotary kiln: Rotary kiln is used for burning process in cement industry. The rotary kiln is a cylindrical steel tube about 3 meters in diameter and 100-120 m in length. Inside of the kiln is lined with refractory bricks and it is held in horizontally inclined position (0.3-0.6 degree). It can rotate at a rate of revolution per minute (1 rpm). Air- fuel mixture is injected from the bottom which fires the inside of the kiln and it will provide a maximum temperature in the range of 1700 degree Celsius and this will keep different temperature ranges from top to bottom of the rotary kiln.

2. Processes: The raw mix or the slurry is injected at the upper end of the rotary kiln and hot flames are forced in to the kiln from the lower end. Due to the slightly inclined position and slow rotation of the kiln, the mixture introduced at the top is slowly moves towards the lower portion of the kiln, the mixture introduced at the top is slowly moves towards the lower portion of the kiln through different temperature ranges.

a. Drying zone: The upper portion of the kiln is the furnace where the temperature is around 400 degree Celsius and the slurry or mixture will dry up properly there.

b. Calcination zone or Nodule zone: When the dried mixture reaches the central portion, where the temperature is around 100o degree Celsius, the lime stone of the mix undergone calcinations to form lime (CaO) and the mass becomes small lumps called nodules or granules. The carbon dioxide produced during calcinations escapes through the chimney as flue gas.

CaCo3 à CaO +CO2

c. Clinkering zone: Thus the calcined nodular mass reaches the central portion, where the temperature is around 1700 degree celcius. At this temperature, the lime CaO reacts with other constituents like silica, alumina, iron etc., to form complex molecules of aluminates and silicates. Then the complex molecules fuse together to form grayish colour small stones like size called clinkers. The hot clinkers are then moved to a rotating cooling cylinder where a blast of air cools the clinkers. The cooled clinkers are then collected in a moving trolley below the cooling chamber.

2CaO + SiO2 -àCa2SiO4 or C2S

3CaO +SiO2- à Ca3SiO5 or C3S

3CaO + Al2O3 à Ca3Al2O6 or C3A

4CaO + Al2O3 + Fe2O3 à Ca4Al2Fe2O10 or C4AF

3. Grinding: The cooled grayish colored clinkers are then ground well in a tube mill or ball mill. Finally powdered gypsum is added proportionally during grinding.

4. Packing: The finely powdered cement material is then fed into automatic packing machines where is get packed.

FUNCTIONING OF GYPSUM IN CEMENT

When cement is mixed with water, the plastic mass or slurry becomes quite stiff within a short time. This is due to the immediate hydration of its constituents tricalcium aluminates Ca3Al2O6.6H2O forming soluble crystalline hexahydrate of tricalcium aluminates. This will prevent hydration or hydrolysis of other constituents thereby preventing a good setting and hardening of the entire mass. This type of initial set is known as flash set.

Ca3Al2O6 + 6H2O -à Ca3Al2O6.6H2O

The presence of gypsum retards this initial set by replacing with hydrated tricalcium aluminates to form insoluble tricalcium sulpho aluminates which will not prevent hydration of hydrolysis reaction of other constituents. Therefore gypsum promotes a high degree setting and hardening of cement.

Ca3Al2O6 + 6H2O+CaSo4.2H2O-àCa3Al2O6.xCaSo4.7H2O

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Written by

Pankaj Kumar

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